Average is best. As always in life, too much or too little of anything is disconcerting. Even here there are two different trends that are pulling in different directions. A wet battery can withstand a lot of punishment as you can always top up the water. On the other hand, a pure lead AGM battery can withstand extremely high loading current. Old logic is becoming open to question, but only in part. Modern battery chargers with more control and cleaner direct current also widen the permitted field of application.
An old rule of thumb says that the recommended size of the battery charger can be worked out by multiplying the capacity of the battery by 0.1 to give the recommended current in Ampere. However, with a modern CTEK battery charger you can charge with a much more powerful model as the current is so clean and the voltage is really well controlled. Charging with a current that is four times greater than the old recommendation is normally no problem with a CTEK battery charger.
A battery charger that is too small will take far too long to charge of course, so much so that it sometimes becomes impractical. A floor cleaning machine or wheelchair has perhaps eight hours before you can use the equipment again, at which time the user wants a battery at its full capacity. On the other hand, the time when the battery is at full voltage is shorter for a small battery than for a large one. This may seem like a paradox, and you need to look at the charge graph to be able to fully understand.
The small battery charger has 13.5 hours to reach full voltage and will have achieved almost ninety percent charge. From here to full charge, however, takes only five hours. The large battery is up at full voltage after six hours but is only at 80 % of its capacity and requires a further six hours to reach full charge. You could also look upon this as if you simply discharged 10 % from the battery, the small battery charger is then just as quick as the battery charger that is twice the size, as it is the battery that dictates the current flow.
A battery charger that is too large is unnecessary for two reasons. Firstly, heating the battery is greater which is unfortunate. The ripple in the battery charger also has an affect here, i.e. how unclean the current is. A battery charger with high ripple, which is common in most linear battery chargers, heats the battery more, which means the current must be kept down. A modern switch mode battery charger has a ripple that is so low that the battery charger can be used for batteries that are smaller than you might expect. The CTEK MULTI XS 3600 with 0.8A charge current has been used for a 1.2Ah battery with only 2 degree temperature rise. A normal maximum temperature rise is considered to be three degrees, which means that CTEK battery chargers cover an extremely wide range. It is possible, of course, to estimate the amount of high current ripple that contributes to heating the battery. A simple rule of thumb is that a traditional linear battery charger with 400% current ripple gives around four times as much heat to the battery compared to a modern battery charger with the technique that CTEK applies.